(1) Field of the Invention
The present invention relates to a method for installing an optical fibre unit in a tube by a pulling rope, for example for connecting end user premises to a telecommunication network.
(2) Description of the Related Art
As “optical fibre unit” it is meant a single optical fibre, optionally buffered, or a plurality of optical fibres assembled to form a multi-fibre unit, optionally contained in a micromodule.
Telecommunication cabling of a multiuser area (for example a multifloor building or a residential area) can be made by extracting optical fibre units from a cable (for example a riser cable) to reach the end user premises by inserting the fibre unit into a tube, generally already laid in the building. Said tube also serves to protect the optical fibre unit, therefore in the following shall referred to as “protective tube”.
A known installation method provides for attaching the optical fibre unit to a pulling rope previously set into the protective tube, as described, for example by WO 97/03376.
The attaching method should be easy to be performed by the installer, and establish a link between optical fibre unit and pulling rope strong enough to withstand the pulling force exerted to drag the optical fibre unit to the end user premise, or to a branching/splicing termination box, through a path possibly comprising sharp bends.
Specifically, WO 97/03376 relates to a method of installing an optical fibre unit in a tube by attaching a pulling member to one end thereof and exerting a pulling force thereon. The pulling member may be attached to fibre unit by a braided sheath, or the pulling member itself is a braided tube. It is necessary that there should be a suitable means for attaching the trailing end of the pulling member to the leading end of the fibre unit. A suitable means of attaching the pulling member to the fibre unit can conveniently be met by a braided sheath. Small diameter braided sheaths are widely available for an entirely unrelated purpose, namely leaders as in fly fishing, and such leaders can be used, without modification, in the method. The ends of the pulling member and optical fibre unit are each inserted in a respective end of the braided sheath. The friction between the pulling member and the sheath and between the fibre unit and the sheath tends to elongate the sheath and thereby cause it to contract and grip the ends. However, friction by itself is not normally sufficient, and additional means, for example a small quantity of adhesive, preferably a rapidly setting adhesive such as a cyanoacrylate, may be used to ensure an adequate grip. Fly fishing leaders are also available as individual sheaths which have one open end and one end formed into a loop. Such leaders are particularly convenient in conjunction with fibre units which have one or more threads of Kevlar or other material therein. The pulling member can be inserted into the open end, and the Kevlar can be tied to the loop.
Also the method taught by WO 99/35525 implies the use of an adhesive material. In particular, WO 99/35525 discloses a method of inserting a light transmitting member into a tube, the method comprising: inserting an elongate, flexible, tensile member into the tube; attaching the tensile member adjacent a rearward end thereof to the light transmitting member adjacent a forward end thereof; introducing the forward end of the light transmitting member into the tube; and causing flow of fluid along the tube to cause the tensile member and light transmitting member to move into the tube. In order to form a joint between the pull cord and the optical fibre, a needle-like tube is pushed inside the end of the pull cord. The tube is pushed out through the side of the pull cord and the end of the optical fibre is then pushed fully into the tube. The tube is then removed leaving the end of the optical fibre in place inside the pull cord. The end of the optical fibre is then pulled until it is just fully inside the pull cord, and glue is applied to the pull cord.
The Applicant experienced that attaching methods using adhesive material for establishing the pulling rope/fibre unit link are not pleased by the installers in view of the possible dirt.
Another method for attaching the optical fibre unit to a pulling rope employs a little tube, generally in deformable metal, having a diameter slightly greater than that of the optical fibre unit to be attached and a length of about 5-10 mm. First, the pulling rope is threaded into the little tube, then the optical fibre unit is inserted into tube (and into the pulling rope, when the pulling rope is in form of a braided sheath), finally the tube is crimped on the optical fibre unit. Such method suffers some drawbacks: in particular, the threading of the pulling rope inside the tube is difficult and requires manual skill to the operator, possibly with the aid of practical tricks such as the moistening to squeeze it in a thinner shape, for example when the rope is in form of a braided sheath; moreover, crimping a round object (the little tube) around another round object (the optical fibre unit when it is a single optical fibre or a plurality of optical fibres housed in a micromodule) can result in poor linking of the pulling rope to the optical fibre unit or may require the use of special pincers in order to provide a coupling strong enough to allow the optical fibre unit pulling through the tube.